This invention is generally related to optical sensors, and, more particularly, to optical sensors used for short-ranging.
In general electro-optical ranging systems in the past have used radiant energy to determine the distance between two objects. A first object transmits the radiant energy and a second object such as a target reflects a portion of the transmitted energy back to the first object wherein a receiver responds to the returned portion of the transmitted energy. A timing means within the first object is able to note the elapsed time between transmission and reception so as to be able to arrive at the range between the objects. The first object having the transmitting and receiving means therein may include separate means such as a light emitting diode for transmitting the radiant energy and a photo-sensitive semiconductor for receiving the returned energy, or have the means combined wherein the transmitting and receiving means is a transducer such as a laser diode adapted to function as both means. There are clearly advantages and disadvantages to both ways. When separated means are employed, one for transmission and one for reception of the radiant energy, the accuracy attained is dependent on the optical train alignment. Further, the receiving means should only be responsive to the wavelength range transmitted by the transmitting means in order to prevent spoofing. To facilitate noise rejection, a filter may be placed in the returned energy path; but the use of such a filter is costly and further limits the acceptance angle of the receiving means. When the first object uses combination means as the transmitter and the receiver, this may be a light emitting diode, for example, The light emitting diode emits radiant energy within a given wavelength range when driven into a conductive condition and also exhibits a conductive condition in response to receipt of the returned energy. The diode is controlled by circuitry which serves to periodically drive the diode into conduction so as to emit energy. The control circuitry and the diode exhibit a dead time after the transmission pulse. This clearly limits the short-ranging capability because the return energy does not cause conduction in the receiving means of the diode and control circuitry during this dead time. This drawback has motivated a search for receiving means that is able to respond to return energy in the time period after the transmission pulse to the end of the dead time, in particular.